Magnetic data storage continues to be the primary, high-performance storage technology in terms of capacity, speed, cost and bytes shipped per year. The success of magnetic storage technology originates from a consistent series of enhancements in capacity and performance combined with significant reductions in price per megabyte. A major technical achievement that has enabled both the rise in storage density and the reduction in storage cost is the magnetic head design and performance. Magnetic storage heads typically comprise transducers for recording and reading localized "bits" on a magnetic medium such as a rotating disk or tape. Head technologies have evolved from ferrite, through thin film inductive to magnetoresistive and giant-magnetoresistive. Successive generations of head technology have enabled a continuous rise in storage density through higher performance and reduced head dimensions. For example, the storage capacity of a hard disk drive is critically dependent upon pole tip dimensions of the write transducer. Contemporary head designs typically have pole tip dimensions on the micron-scale.
In support of these advancements, new manufacturing processes have been developed which are capable of mass producing complex head designs with yields high enough to insure profitability. Magnetic recording heads are manufactured using processes and techniques adapted from integrated device fabrication. Deposition, lithography and etch processes are performed on AlTiC ceramic wafers to form an array of thin film read/write transducers. Wafers are then sliced into bars, whereupon the air bearing surfaces are formed by precision machining and lapping.
Finally, the bars are cut into individual sliders comprising an air-bearing surface and the magnetic head. In this regard, component inspection and testing plays a crucial role in understanding, implementing and controlling advanced process requirements and tolerances. However, even the best processes today cannot produce heads with 100% yield, and therefore it is necessary to inspect magnetic recording heads prior to drive assembly. In particular it is necessary to inspect each head for defects, damage and dimensional conformity before they are attached to a suspension and subsequently integrated into the disk drive assembly.
Optical inspection of recording head surfaces and read/write transducers must be capable of accommodating a variety of structural and material properties. Air bearing surfaces are complex 3-dimensional structures fabricated from granular ceramic composites while head read/write transducers are micron-size metal/insulator structures. For such applications, optical head inspection systems typically comprise semi-automated microscopes that require a human operator to visually detect and classify defective heads and often to operate the microscope itself. Over one billion heads per year are fabricated and inspected in this fashion.
One important aspect of head inspection comprises high-resolution metrology of head structures. Typically head metrology is carried out by imaging the head transducers using a microscope of known magnification and measuring the features of interest. Such methods have sufficient accuracy for larger head structures and indeed are well suited for production inspection because of its high-throughput. However, as head dimensions shrink, noise, systematic errors and limitations of optical imaging become increasingly problematic. Alternatively, it is possible to measure sub-micron head structures using SEM or atomic force microscopes; however, these techniques tend to be too slow and expensive for 100% inspection. Thus, as feature size continues to decrease, the need for precise optical measurement capability increases. Clearly it would be highly desirable to conduct head metrology by optical means in a completely automated manner.